In a weaving operation, weft yarns are picked successively across the warp and are beaten up into the fabric being woven. The pick spacing is dependent on both picking rate, namely, the number of picks per unit time, and take-up speed, namely, the length of the fabric taken up per unit time. That is, the pick spacing varies in direct proportion to picking rate and in inverse proportion to take-up speed.
According to a prior art disclosed in Japanese Patent Publication No. 44-28270, a loom is provided with a take-up motor in addition to a main motor, and the output rotating speed of the take-up motor is controlled on the basis of the difference between the main motor and the take-up motor in output rotating speed so that the output rotating speed of the take-up motor is directly proportional to that of the main motor, in which the proportional constant is variable according to a predetermined program. However, this prior art, basically, is a speed controlling system which employs a tachometer generator to acquire rotating speed signals, and hence the prior art has the following disadvantages.
(1) A weaving bar results from the difference between the main motor and the take-up motor in the first and last transitions of output rotating speed in the inching operation and at the start-up of the loom.
(2) a large difference in characteristics between the tachometer generators causes pick spacing variation between looms.
(3) The operating characteristics of the same loom vary with time due to the time-variation of the tachometer generator in characteristics, and thereby the pick spacing regulating mode of the loom is changed.
(4) The drift of the control characteristics of the speed control system of the analog type due to the variation of temperature or voltage causes the complex variation of pick spacing.